What is the difference between YA and Adults in the Sims 3?

Well, on the physical side of things, adults do have age lines on their faces especially noticeable on the forehead and a bit around the eyes. They are also a teeny bit slower than young adults in their movements. However, the biggest difference I have noticed is that adults are more behaviorally 'settled down' than the younger Sims when it comes to how they handle social and personal relationships as-well-as trauma events.